1. Field of the Invention
This invention relates to a distance measurement apparatus using an electromagnetic wave.
2. Description of the Related Art
There is a prior-art distance measurement apparatus mounted on an automotive vehicle which measures the distance between the present vehicle and a preceding target object such as a preceding vehicle. The prior-art apparatus emits a beam of an electromagnetic wave in a forward direction with respect to the body of the present vehicle. The prior-art apparatus receives an echo beam caused by reflection of the forward electromagnetic-wave beam at a preceding target object. Detection is made about the time interval between the moment of the emission of the forward beam and the moment of the reception of the echo beam. The prior-art apparatus measures the distance between the present vehicle and the preceding target object on the basis of the detected time interval.
In the case where a group of raindrops, a fog, or a spray of water occurs between the present vehicle and a preceding target object, the prior-art apparatus tends to inaccurately measure the distance between the present vehicle and the preceding target object for the reason as follows. A group of raindrops, a fog, and a spray of water reflects the forward electromagnetic-wave beam. The reflection of the forward beam thereat causes a scattered wave beam or an undesired echo beam. The prior-art apparatus responds to the undesired echo beam (the scattered wave beam), and hence fails to accurately measure the distance between the present vehicle and the preceding target object.
A first known way of removing such a problem is to disable a receiver in a distance measurement apparatus for a predetermined time interval after the moment of the emission of a forward electromagnetic-wave beam. The predetermined time interval is chosen to cover a time range during which an undesired echo beam (a scattered wave beam) is expected to reach the receiver. A second known way is to gradually increase the gain of a receiver in a distance measurement apparatus in accordance with the lapse of time from the moment of the emission of a forward electromagnetic-wave beam.
Japanese published unexamined patent application 5-312950 discloses a distance measurement apparatus which emits a leaser beam as a forward beam modulated in accordance with a maximum length code. The apparatus of Japanese application 5-312950 receives an echo beam caused by reflection of the forward beam at an object. The received echo beam is demodulated into a received signal. Detection is made about the phase error between the maximum length code related to the forward beam and a maximum length code represented by the received signal. The distance between the apparatus and the object is calculated from the detected phase error. The apparatus of Japanese application 5-312950 adjusts the gain of a transmitter or the gain of a receiver in response to the magnitude of the received signal. The adjustment-resultant gain is used for a next forward beam or a next echo beam.
The apparatus of Japanese application 5-312950 tends to inaccurately measure the distance between the apparatus and the object in the case where a group of raindrops, a fog, or a spray of water occurs therebetween. A group of raindrops, a fog, and a spray of water reflect the forward beam. The reflection of the forward beam thereat causes a scattered wave beam or an undesired echo beam. When the undesired echo beam (the scattered wave beam) is stronger than a desired echo beam caused by reflection of the forward beam at the object, the apparatus of Japanese application 5-312950 responds to the undesired echo beam and hence fails to accurately measure the distance between the apparatus and the object.
Japanese published unexamined patent application 9-159765 discloses a radar apparatus for a vehicle. The radar apparatus of Japanese application 9-159765 emits a forward light beam, and receives an echo light beam caused by reflection of the forward light beam at a preceding target object. The echo light beam is converted into measurement data. The distance between the present vehicle and the preceding target object is detected on the basis of the measurement data. In the radar apparatus of Japanese application 9-159765, a determination is made about whether or not the measurement data actually has a component corresponding to the echo light beam caused by reflection of the forward light beam at the preceding target object. Reference data is set on the basis of the measurement data which does not have a desired-echo-related component. The reference data represents undesired echo light beams (scattered light beams) caused by reflection of the forward light beam at waterdrops and snow grains. In the radar apparatus of Japanese application 9-159765, current measurement data is corrected in response to the reference data to prevent the undesired echo light beams from causing an error of distance measurement.
It is an object of this invention to provide a distance measurement apparatus which is less adversely affected by a group of raindrops, a fog, and a spray of water.
A first aspect of this invention provides a distance measurement apparatus comprising transmitting means for transmitting a forward electromagnetic wave; receiving means for receiving an echo wave caused by reflection of the forward electromagnetic wave at an object, and converting the received echo wave into a first received signal; scattered-wave detecting means for detecting a scattered wave reaching the receiving means and caused by reflection of the forward electromagnetic wave at an obstacle such as a group of raindrops or a fog which occurs prior to the reflection of the forward electromagnetic wave at the object; cancel-signal generating means for generating a cancel signal to cancel a scattered-wave-corresponding component of the first received signal generated by the receiving means in response to the scattered wave detected by the scattered-wave detecting means; scattered-wave canceling means for removing the scattered-wave-corresponding component from the first received signal in response to the cancel signal generated by the cancel-signal generating means to change the first received signal to a second received signal; and distance calculating means responsive to the second received signal for measuring a time interval between a moment of the transmission of the forward electromagnetic wave by the transmitting means and a moment of the reception of the echo wave by the receiving means, and calculating a distance to the object on the basis of the measured time interval.
A second aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the scattered-wave detecting means comprises means for measuring a strength of the scattered-wave-corresponding component of the first received signal, and means for measuring a delay time between the moment of the transmission of the forward electromagnetic wave by the transmitting means and a moment of the reception of the scattered wave by the receiving means, and wherein the cancel-signal generating means comprises means for generating the cancel signal on the basis of the strength and the delay time measured by the scattered-wave detecting means and at a timing corresponding to the moment of the reception of the scattered wave by the receiving means.
A third aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the transmitting means comprises pseudo-random-noise-code generating means for a pseudo random noise code having a predetermined bit length, means for modulating an electromagnetic wave into a modulated electromagnetic wave in response to the pseudo random noise code, and means for transmitting the modulated electromagnetic wave as the forward electromagnetic wave, and wherein the distance calculating means comprises demodulating means for demodulating the second received signal into a binary signal, correlation calculating means for calculating a value of a correlation between the binary signal and the pseudo random noise code used by the transmitting means, means for detecting a peak-occurrence moment at which the calculated correlation value is maximized, and means for calculating the distance to the object from the detected peak-occurrence moment.
A fourth aspect of this invention is based on the third aspect thereof, and provides a distance measurement apparatus wherein the scattered-wave detecting means comprises means for causing the transmitting means to transmit a forward electromagnetic wave modulated in response to a pseudo random noise code, means for measuring a time interval between a moment of the transmission of the forward electromagnetic wave by the transmitting means and a moment at which the correlation value calculated by the correlation calculating means, delay time setting means for setting the measured time interval as the delay time, means for causing the transmitting means to transmit the forward electromagnetic wave, and signal-level measuring means for detecting a level of the first received signal, generated by the receiving means at a moment the delay time after the moment of the transmission of the forward electromagnetic wave by the transmitting means, as a level of a scattered-wave signal component; and wherein the cancel-signal generating means comprises for delaying the pseudo random noise code by the delay time set by the delay time setting means after the transmitting means starts to transmit the forward electromagnetic wave which is modulated in response to the pseudo random noise code, level adjusting means for controlling a signal level of a pseudo random noise code outputted from the delaying means to the scattered-wave signal level detected by the signal-level measuring means, and means for applying an output signal of the level adjusting means to the scattered-wave canceling means as the cancel signal.
A fifth aspect of this invention is based on the fourth aspect thereof, and provides a distance measurement apparatus wherein the level adjusting means comprises first level adjusting means for adjusting the signal level of the pseudo random noise code outputted from the delaying means at an accuracy comparable to a signal level of white noise, and second level adjusting means for adjusting the signal level of the pseudo random noise code outputted from the delaying means at an accuracy finer than the signal level of white noise; and wherein the signal-level measuring means comprises first level setting means for causing the transmitting means to transmit the forward electromagnetic wave, for causing the delaying means and the scattered-wave canceling means to operate, for changing a quantity of the level adjustment by the first level adjusting means in response to a value of the binary signal generated by the demodulating means the delay time thereafter, and thereby setting the quantity of the level adjustment by the first level adjusting means so that the binary signal generated by the demodulating means will be evenly in its high-level state and its low-level state, and second level setting means for causing the transmitting means to transmit the forward electromagnetic wave after the first level setting means sets the quantity of the level adjustment, for causing the delaying means and the scattered-wave canceling means to operate, for measuring the binary signal generated by the demodulating means a plurality of times, for calculating a number of times the measured binary signal is in one of its high-level state and its low-level state during the plural-times measurement of the binary signal, and for setting a quantity of the level adjustment by the second level adjusting means in response to the calculated number of times so that a level of a signal from which a scattered-wave component has been removed and which is inputted into the demodulating means will be smaller than the signal level of white noise.
A sixth aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the cancel-signal generating means comprises means for limiting a level of the cancel signal to a predetermined upper limit.